If during their Declaration of War phase, the Axis control 3 hexes adjacent to Paris, the German player may announce that Paris has been declared an Open City.
Effects of declaring Paris an Open City
The French player immediately moves all Allied units in Paris to the nearest French hex they can stack in (ignoring ZOCs and enemy-controlled hexes). Any units moved maintain their current facing. In the unlikely event that there is no French hex in which the Allied units can stack, the units are destroyed.
French units may not attack or enter Paris, nor may they fly any air mission to Paris, until after France is vichied or conquered.
Rationale
Paris is actually a very tough city to take. One bad die roll and France can last into 1941, almost certainly dooming the game for Germany. The game is not really about France and it shouldn't be decided this early.
LOC Vichy also has an "Open City" rule but LOC Vichy is a whole rules subsystem complete with many different Vichy outcomes. If you're looking for a lighter variant that ensures that you don't get bogged down in an ahistoric battle for Paris, try this rule.
